The present invention relates generally to energy delivery systems, and more particularly, to a system and method for testing winding and winding connection deformations and/or displacements in a transformer.
Electric utilities and other organizations are responsible for supplying an economic, reliable and safe source of electricity. Three major components are employed in an energy delivery system to provide the electricity to the end user, the generator, the transmission line and the transformer.
Generators are rotating machines operated in a manner such that electricity is created when mechanical energy is used to rotate the generator shaft. A generator rotor is coupled to the shaft, and when the shaft is rotated, thereby rotating the rotor, a voltage and current is caused in the generator stator. One typical form of mechanical energy used to generate electricity is steam, which is passed through a turbine that forces the generator shaft to rotate. Steam is often created by boiling water using coal, natural gas or nuclear fission heat sources. Or, steam may be taken directly from naturally occurring geothermal sources. Other sources of mechanical power employed for rotating a generator rotor may also include hydroelectric power or wind power. Since the end user of the electricity is rarely located near a generator, the electricity generated by the generator must be xe2x80x9ctransportedxe2x80x9d to the end user.
The second major component employed in an energy delivery system is the transmission line. Transmission lines consist of a grouping of wires which connect the generator to the end user. The xe2x80x9camountxe2x80x9d of electricity that a transmission line can carry depends primarily upon the diameter of and number of the conductors (wires) used on the transmission line, and the voltage at which the transmission line is operated at. Typically, transmission lines from the generators employ a relatively high voltage so that a large amount of electricity is economically and reliably transported over long distances to locations where large concentrations of end users are found, such as a city or a large industrial manufacturing plant. Examples of extra high voltage (EHV) and intermediate transmission voltages employed in the industry include, but are not limited to, 500 kilo-volts (kV), 230 kV, 138 kV, 115 kV, 69 kV and 46 kV. Typically, lower transmission line voltages are employed on the transmission line distribution system (such as, but not limited to, 25 kV, 20 kV, 13.8 kV, 12 kV, 4 kV, 480V and 240V) to provide energy to the end user""s premises connection point.
The third major component employed in an energy delivery system is the transformer. The transformer is a device that changes voltage. Generally, voltage from the generator is a lower voltage than used by the transmission lines that transmit the electricity to the end user. Furthermore, the voltage used by the end user is much lower than voltage used by the transmission lines. Thus, the transformer couples elements of an energy delivery system that employ different voltages.
For example, two voltages typically found in a home are 240 volts and 120 volts. An EHV 500 kV transmission line may be delivering power to a city that employs a 230 kV transmission line system to deliver energy to a 13.8 kV distribution system. A 500/230 kV transformer changes voltage from 500 kV to 230 kV, thereby allowing two transmission lines having different operating voltages (500 kV and 230 kV) to be coupled together. Such a transformer has at least two terminals, a 500 kV terminal and a 230 kV terminal. Similarly, a 13.8 kV/240V/120V transformer may be used to convert voltage of the 13.8 kV distribution system to a voltage used in the end user""s home or office. Thus, transformers allow the various voltage generators, transmission lines and distribution lines to be coupled to a home, office or other facility where the end user will be using the electricity.
Transformers come in many different sizes, shapes and constructions. Typically, transformer size (rating) is specified as the product of the maximum voltage and current, as measured from one side of the transformer, that the transformer is capable of converting at a particular operating condition. Such operating conditions include temperature and/or altitude. For example, a 500/230 kV transformer may be rated at 300 MVA (3,000 kilo-volt-amps) when operating at sea level and at 65xc2x0 Celsius rise above ambient. Transformers may be constructed as separately insulated winding transformers or auto transformers, and as single phase or multiple phase transformers. The operating voltages, ratings and winding types of transformers employed in the industry, well known to one skilled in the art, are too numerous to describe in detail here other than to the extent necessary to understand the present deficiencies in the prior art.
All transformers, independent of size, rating and operating voltage, have several common characteristics. First, the transformer is constructed from one or more windings, each winding having a plurality of individual coils arranged and connected in an end-to-end fashion. In some transformers, the winding is made by wrapping a wire around a laminated solid member, called a core. Alternatively, there may be no core. However, in all transformers, the individual windings must be electrically isolated from each other. An insulation material is wrapped around the wires such that when the plurality of coils are made, the metal wires of each winding are physically and electrically separated, or insulated, from each other. Insulation materials wrapped around the windings may vary. Paper, impregnated with oil, is often used. Other types of transformers may use only paper, or may use another suitable material such as a polymeric compound.
Maintaining the electrical insulation between the windings is absolutely essential for the proper operation of a transformer. In the event that the electrical insulation is breached, such that electricity passes from one winding coil across the breach to another winding coil, special protective devices will operate to disconnect the transformer from the electrical system. The devices, by removing electricity applied to the transformer, interrupt the undesirable current flow through the insulation breach to minimize damage to the transformer. This condition is commonly referred to in the industry as a transformer fault.
Transformer faults are undesirable for at least two major reasons. First, end users may become separated from the energy delivery system, thereby loosing their electrical service. Second, transformer faults may result in large magnitudes of current flow, known as fault current, across the breach and through the transformer windings. Also, faults occurring on the energy delivery system at locations relatively close to the transformer may result in large fault currents flowing through the transformer. Often, fault current may be orders of magnitude greater than the highest level of normal operating current that the transformer was designed to carry. Such fault currents may cause severe physical damage to the transformer. For example, a fault current may physically bend portions of the transformer winding (winding deformation) and/or move the windings out of their original position in the transformer (winding displacement). Such winding deformation and/or displacement can cause over-voltage stresses on portions of the winding insulation and exacerbate the process of the naturally occurring deterioration of the winding insulation that occurs over a period of time. The fault current may further increase damage to the insulation, or damage insulation of adjacent windings, thereby increasing the magnitude and severity of the fault. In the most extreme cases, the fault current may cause an ignition in the transformer oil, resulting in a breach of the transformer casing and a subsequent fire or explosion.
Therefore, it is desirable to ensure the integrity of the transformer winding insulation. Once a transformer fault occurs, it is usually too late to minimize transformer damage and to reduce the period of electrical outage. The electric utility industry takes a variety of precautionary steps to ensure the integrity of winding insulation in transformers. One important precautionary step includes periodic testing of the transformer. Various tests are used to predict a probability of a future fault. One test commonly employed in the industry to detect winding deformation and/or displacement is the low voltage impulse test.
To perform a low voltage impulse test according to prior art methods, requires that the transformer be de-energized. The transformer is de-energized by physically isolating the transformer from the energy delivery system. After the test apparatus is coupled to the transformer, an electrical pulse is applied to one terminal of a transformer. That is, a signal or pulse is applied to a selected input winding of the transformer. The signal or pulse on a selected output side of the transformer (output winding) is then measured. The input and output signals or pulses are analyzed using a variety of techniques. One analysis technique is to perform a frequency response analysis (FRA) which measures one characteristic of the input and output signals over a predetermined frequency range. One commonly employed technique is to process the measured input and output signals or pulses by applying a fast Fourier transform (FFT) to the signals. The FFT of the output signal is divided by the FFT of the input signal and the resultant admittance, as a function of frequency, may be plotted for the transformer. The input signal or input pulse may be applied to the high voltage, low voltage, neutral or other suitable terminal that is available on the transformer. The output signal is taken from another selected terminal on the transformer. For example, a low voltage pulse is applied to the high voltage terminal of the transformer winding and the output pulse is measured on the low voltage terminal of the transformer. Such a test is commonly known as a low voltage impulse test because the voltage of the applied input signal or pulse is much less than the impulse voltages used to test for dielectric integrity in a high voltage laboratory or at the transformer manufacturing site. When a series of identical pulses are applied to the transformer winding, in accordance with prior art transformer testing procedures, and the resultant measurements are averaged together, the resultant plot is often referred to as the transfer function or characteristic signature of the transformer winding configuration being tested.
In a static situation, a test engineer could reasonably expect that the characteristic signature of the transformer winding would not significantly change with time. For example, the test engineer could reasonably expect that a transformer winding tested one year after being placed in service, assuming that nothing has changed within the transformer during that year, could be tested and have a transfer function that would substantially match the transfer function taken a year earlier.
However, static conditions rarely occur in the field. Each time current flow is adjusted in the transformer, mechanical stress in the windings change. Abrupt changes in current flow can occur when a portion of the electrical transmission system is reconfigured by switching, when lightning strikes the transmission system, or when faults occur on components of the transmission system, that are nearby the transformer. Many other events may also cause abrupt changes in current through the transformer on a regular and frequent basis. This is a basic reality of the operation of the electric system. Transformer windings are designed to accommodate a number of reasonable magnitudes of abrupt current change over the operating life of the transformer. Yet, abrupt current changes in excess of the design limits are occasionally encountered. When these conditions occur, the windings may permanently bend from their original position, hereinafter referred to as winding deformation. Or, the windings may move slightly from their originally installed position, hereinafter referred to as winding displacement. Winding deformation and displacement may stress, crack and/or otherwise impair the insulation around the windings. Furthermore, the impairment caused to the winding insulation by each abrupt change in current is cumulative. That is, the damage is not self-repairing or healing. Eventually, the cumulative damage may become sufficient to cause a breach in the insulation. Then, a fault will occur and the transformer will become damaged, thereby requiring the transformer to be taken out of service for repair or retirement.
Additionally, winding deformation and/or displacement may alter the voltage gradient around the bent portion of the coils. If the winding deformation and/or displacement decreases the gap between two adjacent winding coils, the voltage gradient may become more concentrated around the bend. The increase in the voltage gradient may be sufficient to breach the insulation, thereby causing a fault. Or, the increased voltage gradient may cause a temperature increase around the deformed and/or displaced portion of the coils. The increased temperature increases the rate of degradation of the winding insulation. In an oil filled transformer, the temperature increase may alter the properties of the transformer oil, and possibly result in electrical partial discharge which in turn results in the formation of undesirable gasses.
Thus, periodic testing is performed to determine and/or estimate the amount of cumulative damage to the transformer resulting from the normal (and abnormal) day-to-day operating conditions that the transformer has been subjected to. If the tests indicate potential problems, the transformer can be scheduled for maintenance, or replaced if necessary, in a timely and controlled manner that results in the least disruption in service to the end users. Furthermore, transformers. are very expensive pieces of equipment, thus repairing a transformer before permanent damage occurs is desirable.
Prior art low voltage impulse tests present many unique problems. One problem is that the transformer must be de-energized from the energy delivery system (by physically isolating the transformer). Another significant problem is that a precise, repeatable input testing signal or pulse must be applied to the input terminal of the tested transformer winding when prior art frequency response analysis techniques are used to measure the frequency response of the transformer winding. If the applied input test signals/pulses are not identical to each other, the resultant characteristic signature of the tested transformer windings will not be accurate. In addition, the time delay between pulse applications should be constant to prevent distortion of the characteristic signature. For example, if the pulse intervals are not constant, the energy storage remaining in the transformer winding configuration will be different between pulses, thus altering the load impedance of the transformer and therefore, changing the parameters of the applied pulse. Furthermore, test signal/pulse generators or test pulse generators capable of providing such exact and repetitive input signals or pulses are expensive. Also, personnel must take time from their otherwise busy schedule to de-energize the transformer and to perform the tests.
Therefore, it is desirable to have a valid and reliable testing system and method that does not require the transformer to be de-energized. Also, it would be desirable for the test equipment to be inexpensive, to be easily portable, and to be easily implemented in the field where the transformer is located. Furthermore, it would be desirable to have the test signal/pulse generator configured to provide a wide variety of test signals/pulses suitable for testing a wide variety of transformers.
The present invention provides a system and method for determining a characteristic signature, H(f), of an energized winding. Briefly described, in architecture, the system and method can be implemented as follows.
In one embodiment, a sensor detects incoming voltage pulses due to abrupt changes in current or voltage originating elsewhere on the energy delivery system. The above described incoming pulses enter the transformer winding that is monitored by the sensor. Another sensor detects an output pulse after the applied input pulse has propagated through the monitored winding. Spectral densities are determined from these detected input and output pulses. However, the electrical characteristics of these pulses, such as the current, voltage, frequency, wave shape and/or energy are unpredictable and vary randomly from pulse to pulse. Not all pulses will have sufficient energy to generate useable information that can be used to calculate spectral densities for all the frequencies of interest. Some pulses may have sufficient energy so that the spectral densities for all of the frequencies of interest are calculated. Other pulses will have sufficient energy in some frequencies so that spectral densities for some portions of the frequencies of interest are calculated. The present invention, an on-line winding test unit, monitors a winding and records the input and output pulses. Logic is executed that analyzes the input and output pulses by way of spectral densities to identify useable H(f) data, which is further processed to build a transfer function from the pieces of usable data. When a sufficient record of useable H(f) data portions are accumulated, a complete characteristic signature, H(f), for the monitored winding is constructed. Winding deformation and/or displacement can be determined by comparing the most recent computed characteristic signature with an earlier characteristic signature.
In another embodiment, data corresponding to the detected input pulse, and data corresponding to the detected output pulse, is stored in a memory. Incoming data corresponding to the detected input pulse and the detected output pulse are allocated to a plurality of frequency bins by the digitization process for later analysis. The frequency bins are defined by the digital sampling rate and the data record length and can be refined further with zero padding. This produces a predefined frequency bin width in the frequency domain. Thus, the plurality of frequency bins provides a convenient way to partition the frequency range of the characteristic signal. For example, that frequency domain portion of the detected input pulse and the detected output pulse over the range of 1,000 kilo-Hertz (kHz) to 1,010 kHz would be xe2x80x9cassignedxe2x80x9d to the bin designated for the frequency range of 1,000 kHz to 1,010 kHz, thus creating a data point for that 10 kHz wide frequency bin. Likewise, the remaining frequency domain data of the detected input pulse and the detected output pulse are assigned to the plurality of frequency bins according to the frequency of the data, thereby creating a plurality of data points for the respective frequency bins. In one embodiment, the frequency bin bandwidth is 3 kHz.
In another embodiment, after the input and output waveforms are digitized using a predetermined sampling rate and record length, the required spectral densities are calculated. First, the auto-spectral density (Gxx) is calculated. Gxx is defined by the complex conjugate of the FFT of the input pulse times the FFT of the input pulse. Second, the cross-spectral density (Gxy) is calculated. Gxy is defined by the complex conjugate of the FFT of the input pulse times the FFT of the output pulse. The logic then calculates the characteristic signature [H(f)] for the winding such that H(f) equals the average of the Gxy""s divided by the average of the Gxx""s for the respective pairs of input and output pulses. This H(f) representation is chosen for noise rejection in the output signal. For example, only the output components which are correlated with the input pulse are accepted. Anything extra, such as un-correlated noise, is rejected by definition. As opposed to the prior art, spectral densities work best when the input pulses are slightly different or very different in shape.
To determine the acceptability of H(f) for each frequency bin, a coherence function xcex32xy(f) is calculated for each bin, according to the equation below:
xcex32xy(f)=|Gxy(f)|2/Gxx(f)Gyy(f)
The coherence function xcex32xy(f) is a real valued function having a magnitude ranging from 0 to 1. A value of 1 would indicate a perfect linear relationship from the input pulse to the detected output pulse. In addition, a value of 1 indicates that there was sufficient input energy in the applied pulse such that the H(f) for the frequencies of interest could be accurately calculated. A value between 0.4 and 1.0 indicates that the H(f) data points in the bin have some reasonable degree of accuracy. In one embodiment, the H(f) data points are weighted more heavily as the coherence value increases from 0.4 to 1.0 after the calculation of characteristic signature of the monitored winding. Data points having a coherence value of less than 0.4 are considered as invalid data (not having sufficient energy at the frequency of that bin) and are rejected. A value of 0 would indicate a complete non-linear relationship between the input and detected output pulses or not enough input energy for that frequency bin calculation.
Additionally, a random error function Er|H(f)| for each bin is calculated, according to the equation below:
Er|H(f)|=[1xe2x88x92xcex32xy(f)]xc2xd/|xcex3xy(f)|(2nd)xc2xd
In one embodiment, the random error function is calculated from a previously accumulated data base known to be valid. The random error, Er|H(f)|, provides a statistical analysis of the data and defines a 95% confidence interval for the H(f) data that is graphed over a frequency range of interest. The random error for the frequency bin(s) of the present data evaluation period can be compared to the random error for the same frequency bin(s) on a past data evaluation period for the same winding. If the random error increases significantly for the 95% confidence interval during a follow-up period, then there is a problem with the equipment set-up for this particular test. For example, there could be problems such as loss of input pulse, open leads, or open connections, or wrong connections etc. with the test set. A real change in H(f) due to winding displacement will not significantly change the random error calculation for a given test configuration.
After a suitable number of input pulses have propagated through the monitored winding such that a pre-determined number of frequency bins contain a suitable amount of valid data for analysis, in one embodiment, the processor automatically calculates and stores the characteristic signature of the monitored winding. The H(f) data points for a selected number of frequency bins are combined to create a characteristic signature over the frequency range of interest. Many of the xe2x80x9cemptyxe2x80x9d or unusable frequency bins can be omitted from the H(f) construction and still provide a very good H(f) representation. For example, most winding deformation and/or dislocation will alter the H(f) over many adjacent frequency bins, so omitting frequency bins will not significantly compromise the H(f) construction. In one embodiment, the frequency range of interest is from approximately 3.0 kilo-Hertz (kHz) to approximately 5.0 mega-Hertz (MHz).
One embodiment of the on-line winding test unit includes logic that automatically analyzes the coherence function and the random error function of two separate H(f)""s. This information is used in comparing a previously calculated H(f) to the present H(f) to determine a comparison number. The overall comparison number, which is based on the sum of the individual frequency bin comparisons, is weighted on the value of the two respective error functions for the two H(f)""s being compared. For example, if the individual H(f) point errors are large (but still about the same magnitude), the calculated difference between the two H(f)""s for this frequency bin is automatically reduced. Based upon pre-defined empirical test data, the calculated comparison number is displayed with a color coded system. For example, if the comparison displays a number in green, then little or no change has occurred between the two compared H(f)""s of the monitored transformer winding. If the comparison numbers are displayed in yellow, then the yellow color indicates that some change has occurred. However, such changes denoted by the yellow color may be considered to be associated with temperature differences between the data bases, differing conditions of transformer oil between the data bases, normal aging of the insulation that occurs over a number of years, and/or some other change in the transformer not necessarily related to winding deformation or displacement. If the comparison numbers are displayed in red, an indication of a significant change in the monitored winding is indicated, such as a possible winding deformation or displacement. Should the comparison calculations be well into the red zone, the monitored winding should be inspected and scheduled for possible repair in the near future. Therefore, this embodiment of the on-line winding test unit is particularly well-suited for use by an individual who is not necessarily skilled in the art of analyzing transformers, such as a technician or other maintenance personnel.
Another embodiment includes a pulse or signal generator. The pulse or signal generator generates a pulse having characteristics that provide useable data in the frequencies of interest. This embodiment is particularly suited for supplementing the data collected from incoming pulses originating on the energy delivery system.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.